Other current assets (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid expenses	$ 1,203	$ 1,827
Refundable research and development tax credits	0	809
Security deposits and other receivables	634	622
Interest receivables	179	96
Other current assets	$ 2,016	$ 3,354